The present invention relates to a novel axial/radial flow converter, preferably—but not exclusively—for use as an ammonia converter.
Ammonia converters are complicated due to the fact that the synthesis of ammonia from nitrogen and hydrogen gas (in an approximate ratio of 1:3) is exothermic, and the reactions take place at high temperatures and pressures. Thus, interstage cooling is generally used between a series of catalyst zones to maintain kinetic and equilibrium conditions appropriate for optimum conversion efficiency. There must also be provisions made for servicing the catalyst zones, e.g. periodically removing and replacing catalyst when it loses its effectiveness.
Because ammonia converters are complicated, but also very important pieces of equipment, many efforts are made to improve their efficiency. Thus, US 2004/0096370 discloses a split-flow vertical ammonia converter, in which a fixed-bed catalyst zone is configured into two mechanically separated catalyst volumes and two gas streams operating in parallel. This design maintains the ratio of gas flow to catalyst volume so that there is no catalyst effectiveness loss. The catalyst beds and gas flow paths are configured so that the gas flow is downwards through each catalyst volume.
According to US 2008/0014137, ammonia is produced in a converter in which pseudo-isothermal conditions can be approached by convection cooling of a reaction zone by positioning at least a portion of said zone in indirect contact with a flow of hot gas, such as exhaust gas or pre-heated air.
The use of axial-radial flow reactors in synthesis processes is not novel in itself. It is e.g. disclosed in U.S. Pat. No. 5,427,760, which describes axial-radial reactors in the Braun synloop with external heat sink. In U.S. Pat. No. 4,372,920, an axial-radial reactor for use in heterogeneous synthesis is described, and U.S. Pat. No. 5,352,428 deals with high-conversion ammonia synthesis. FIG. 4 of the latter US patent is an illustration of an axial-radial flow reactor suitable for use in the apparatus and process described.
US 2002/0102192 A1 describes a catalytic reactor wherein axial-radial flow may be achieved with the consequent advantages of a reduced pressure differential, but without any “complex reactor internals”. The reactor has inlet and outlet ports and a bed of particulate catalyst disposed round a central region communicating with one of the ports and presenting less resistance to flow than the catalyst particles. The central region within the catalyst bed has a height equal to at least a major part of the height of the catalyst bed, and the exterior surface of the catalyst bed less than that of the reactor, thus leaving a space between the exterior surface of the catalyst bed and the interior walls of the reactor, said space being filled with a particulate material with less resistance to flow than the catalyst particles.
In EP 2 167 226 B1, a wall system for catalytic beds of reactors for heterogeneous synthesis of chemical compounds is disclosed. The reactors are equipped with fixed catalyst beds crossed by a gaseous flow of synthesis gas, particularly with axial-radial flow. The design may resemble that of the present invention, but the canister concept is not envisaged.
A multi-bed catalytic converter with inter-bed heat exchangers, comprising a plurality of superimposed catalytic beds and a common heat exchanger, is disclosed in EP 2 759 338 A1. The design of this converter does not have much in common with the design of the axial/radial flow converter of the present invention.
Finally, US 2004/0204507 describes a cooled axial/radial flow converter comprising an annular catalyst bed and a plurality of cooling panels arranged in a radial pattern inside the catalyst bed and surrounding a central pipe. The catalyst bed and the shell of the converter forms an outer annulus through which a process gas is passed to the catalyst bed. The process gas flows in axial-radial direction through the catalyst bed and is subsequently collected in the central pipe. The axial/radial flow converter of the present invention differs from that of the US application in that the catalyst bed is divided into a number of identical modules stacked on top of each other and also in that the process gas is passed through the cooling panels to pre-heat the gas.
When low pressure drop is required in a fixed bed catalytic converter, a radial flow type converter is often selected. However, in special cases, such as cooled catalyst bed, catalyst shrinkage or catalyst particles having low strength combined with a high catalyst bed, this solution is not practical, and instead inter-bed cooling or parallel reactors must be selected.